FIGS. 1 and 2 illustrate a selected stage of the prior art fabrication process of a non-volatile MRAM array on a wafer. The stack of thin film layers are deposited on a substrate containing CMOS circuitry (not shown) and landing pads for electrical connections to the bottom electrode (BE) for the magnetic tunnel junction (MTJ) elements. A typical magnetic tunnel junction (MTJ) cell consists of a bottom electrode (BE) and seed layers, followed by an anti-ferromagnetic pinning layer (AFM). The composition of AFM layer is typically alloys of IrMn or PtMn. A fixed magnetic layer is deposited on top of the AFM layer. The fixed magnetic layer typically has the structure of a synthetic anti-ferromagnetic layer with two ferromagnetic layers made from compounds of Co and Fe, separated by a very thin Ru layer to induce anti-ferromagnetic coupling between the two ferromagnetic sub layers. A tunnel barrier is deposited on top of the fixed layer. The composition of tunnel barrier layer is preferably MgO. The tunnel barrier separated the fixed layer from a free magnetic layer preferably made of magnetic materials of ferromagnetic elements Co, Fe, and Ni. Free layer may also contain up to 20 atomic % of B. The AFM layer, fixed magnetic layer, tunnel barrier layer, and free magnetic layer make up the important parts of the MTJ.
The fixed magnetic layer has a magnetic moment direction pinned into a single direction through exchange coupling to the adjacent AFM layer. The magnetic moment direction of the free magnetic layer can be changed between the parallel and anti-parallel direction with respect to the fixed magnetic layer. The tunnel barrier layer allows electrons to tunnel between the fixed magnetic layer and the free magnetic layer. When the free magnetic layer's magnetic moment is parallel with the fixed layer's magnetic moment, the resistance to electron flow through tunnel barrier layer is lower; and when the free layer's magnetic moment is anti-parallel with the fixed layer's magnetic moment, the resistance to electron flow through tunnel barrier layer is higher. This difference in resistance is also known as “tunnel magneto-resistance” or TMR.
On top of the MTJ/bottom electrode stack, a top electrode and dielectric layers are deposited. The dielectric layer functions as an etch mask during the patterning of the MTJ element and bottom electrode. Typically, the dielectric etch mask has the shape of a post or pillar. In conventional lithography, the control of the critical dimensions of the post shape is rather limited and hence strongly affects the size control of the final MTJ element. To alleviate this critical dimension control in two dimensions, a fabrication process involving two photolithography steps and two etch steps have been previously been proposed (U.S. Pat. No. 7,863,060 to Belen, et al. (Jan. 4, 2011)). However, the thickness of the dielectric layer in the prior art limits the resolution of the patterning process and introduces a large topography and gap to be filled prior to any subsequent patterning step. The current invention addresses this deficiency by using a very thin dielectric mask, as well as provide methods to reduce the size of the MTJ element and to self-align the bitlines to the MTJ elements.